Could the traditional advice for this exercise mainstay be leading your clients astray? Research provides answers.
“Hey, keep your knees behind your toes when you squat!”
“Deep squats are bad for the knees!”
“My doctor told me I should not squat anymore.”
“You should never let the knees cave in or out during a squat.”
Chances are you’ve heard this advice and maybe even given it to your clients. I know that for many years in my career I’ve been guilty of making similar recommendations to clients from all walks of life. The problem is, where did this advice come from? Is it valid and who is it valid for? What principles should we follow when doing or teaching one of the most popular exercises on the planet?
This article will share much of the latest research about the science and application of squats and will help separate fact from myth. I’ll then explore squat training fundamentals and provide strategies for personalizing squats so they match clients’ abilities and goals.
Keep the Knees Behind the Toes?
Though “Keep the knees behind the toes” is a popular cuing strategy when coaching squats, the source of this cue—along with the purpose it serves—is up for debate. Picture a group session with one instructor and 30 students: If squats cause knee pain for four or five people, it’s reasonable for the instructor to shout this cue to help clients in pain distribute force away from the knees without disrupting the flow of the entire class.
But does this cue actually promote a healthy squat, or does it simply isolate force to a different region of the body? Let’s look at three kinds of squats, shown in Figures 1–3:
In an even squat, ankles, knees and hips flex at the same time during the descent. For most people, the torso and lower leg will be parallel.
In an anterior-loaded squat, the ankles and knees usually flex faster and further than the hips.
In a posterior squat, the hips bend faster and/or further than the knees.
One of the body’s primary strategies for a dynamic movement like a squat is to mitigate stress by distributing it more evenly through the system and to use as little energy as necessary to get the job done. During a squat, the body uses synergistic muscle relationships and joint synchronization to produce, stabilize and transfer force through the system.
McKean & Burkett (2012a) have studied the squat extensively and found that “restricting the movement of the knee during a squat will alter the movement sequence and hence place undue strain on segmental joints during the squat.” If we consciously discourage the knee from flexing as we try to lower the hips, we simply displace more tension through the posterior structures and disrupt normal joint synchronization. As a result, more energy is expended, and there is more force through the lower-back area, potentially creating a complication elsewhere in the body.
Three main factors dictate whether the knees will cross the toes during a squat, allowing professionals to personalize coaching strategies:
- Tibia-femur-torso length ratio.People with a long femur and/or torso will squat with the knees crossing the toes to maintain equal force distribution during the squat (McKean & Burkett 2012b). Otherwise, those with long femurs and/or torsos might do knee-dominant or hip-dominant squats.
- Mobility of the ankles and hips. Clients will need adequate flexibility of the ankle complex and the hip if the squat’s goal is to maximize force distribution. Lack of flexibility also impairs joint synchronization and timing, leading to excessive wear through the soft tissues of the knee and low back (Kritz, Cronin & Hume 2009). This is one of the reasons why many power-lifting shoes have an elevated heel to allow more ankle motion and help maintain an upright torso.
- Loaded vs. unloaded squat, and load placement. The amount of load added during a squat and where that load is carried and driven can influence the squat’s dynamics. For example, performing a squat with weight held anterior to the chest increases the dominance of the hip extensors (Lynn & Noffal 2012).
Must Knees Stay in Line With the Toes?
If the knees were pure hinge joints (like the hinges on a door), we would expect them to consistently line up with the toes during a squat. Although the structure of the knee certainly favors a unidirectional orientation, it’s important to recognize that this joint does in fact move in all three planes of motion—especially in concert with the hip and the ankle (Gray 2003). These factors determine whether the knees should line up with the second and third metatarsals:
- Q-angle of the hip
- amount of load
- length relationship of the tibia and femur
- ankle and hip mobility
- intent when executing the exercise
Looking at Figures 4 and 5, you may notice that the orientation of the acetabulum in the pelvis or the angle in which the femur approximates the acetabulum can differ from person to person. This can dramatically influence whether your client has a bias to squat with a slight knee valgus (knees caving inward) or a slight knee varus (knees pushing outward). Depending on the height of the squat, this may cause the femur to track in or out—or sometimes to do both. McKean and Burkett (2012a) conclude that knee action is required: “[It] should not be discouraged if the knees move both in and out slightly during the descent, and [this] is not necessarily related to muscular weakness but may be linked with the overall movement strategy.” This also plays to the notion that the ideal foot position for heavy squats depends on the individual, and we should avoid forcing people to squat with one type of foot stance.
The angle at which the acetabulum sits in the pelvis can bias which plane of motion the hips move through during a squat.
The length and angle of the femoral neck (among other structural traits of the femur) can also create bias in motion.
Three-dimensional squats challenge the body’s ability to squat through a variety of directions, ranges of motion and starting positions. A common sporting example is the hockey goalie. In Figure 6, notice how the goalie’s knees cave in and internally rotate to block a shot between the legs. If the knees are purposefully driven in and out during a squat, the lower-extremity tissues must have enough strength to tolerate the direction and amount of force running through them.
A dynamic squat is not destined to create knee pain or excessive wear and tear so long as proper training and diet ensure the quality of the tissue matrix to withstand such forces. This adaptive process is called mechanotransduction, which fortifies stronger tissue and bone to withstand higher levels of force in the direction and point of application of force on the body. The key take-home is that neither knee valgus nor knee varus is inherently bad when it is desired and the body has the tensional framework to dissipate force.
Conditioning the soft tissues to handle unique directional forces and joint positions is paramount for an athlete like this hockey goalie.
Are Deep Squats Bad for the Knees?
It is understandable to assume that the deeper the knee flexes, the more pressure this puts on the soft-tissue structures surrounding the knee. However, is this true? And is more “pressure” or force a bad thing?
Interestingly enough, force applied to the anterior cruciate ligament and the posterior cruciate ligament during a squat actually diminishes in the deeper portions of the squat. In a study analyzing load on the knee at various squat heights, Hartmann, Wirth & Klusemann (2013) say that concerns over the apparent higher risk for chondromalacia, osteoarthritis and osteochondritis in deep squats are unfounded. In fact, shallower squats actually expose the knee to greater compressive forces.
If you’re looking to explore a deep squat, recognize these three basic squatting principles:
- Load and joint freedom of motion have an inverse relationship. This means as load increases, freedom of motion through the joints decreases and vice versa. In other words, the heavier the load, the stricter we want to be about enforcing ideal form.
- Rhythm and timing (joint synchronization) are things to look for. Ideally, the ankles, knees and hips will all flex and extend congruently. Limitations in ankle and hip mobility can alter joint synchronization and inhibit a deep squat. Therefore, a great place to start conditioning a deep range is to elevate the heels or hold onto a TRX® Suspension Trainer™ to counterbalance the hips. Combine this with mobility strategies and clients will be on their way.
- Pain is a signal to modify the squat. If squatting causes pain, modify the exercise by altering the footprint, range of motion, speed or direction in which the pelvis is being driven. If pain persists, then refer to a specialist.
Webster's Dictionary defines variability from a biological perspective as “the power possessed by living organisms, both animal and vegetable, of adapting themselves to modifications or changes in their environment, thus possibly giving rise to ultimate variation of structure or function.” From a mechanical standpoint, reduced variability is known to cause repetitive stress injuries, while an optimal movement system has the capacity to perform a given task in a variety of ways (Harbourne & Stergiou 2009). This enhances our entire being, from our heart and nervous system to our connective tissues and bones. And for bodybuilders, there may be benefits to including variable movement strategies for increased strength and hypertrophy (Fonseca et al. 2014).
A standard personal training certification covers most of the training principles for traditional, sagittal-plane, heavy-loaded squats. What’s missing in certification texts are the benefits and rules of incorporating variable squats. (See Table 1 for coaching tips on variable squats and Table 2 for a comparison of traditional vs. variable squats.)
Variable squats offer a host of potential benefits when performed in the right environment:
- Exploring new positions through different planes of motion enhances the nervous system’s motor control. Simply put, learning how to squat in a variety of ways encourages the nervous system to find the optimal way to disperse forces through the system in multiple directions and positions.
- Bone density may increase to tolerate variable directional force (see mechanotransduction).
- Variable forces enhance the connective tissue matrix, improving shape stability, tissue resiliency and joint integrity (Myers 2011).
- Withstanding different lines of force requires greater intra- and inter-muscular coordination, potentially enhancing strength gains and hypertrophy (Fonseca et al. 2014).
Squat to low-to-medium range of motion with hands together out in front of chest. While holding this range of motion, drive left hand to left and behind body, allowing thoracic spine, hips and ankles to rotate to disperse force through body. If ankle or hip mobility is limited, be sure to pivot trail leg.
Client assumes staggered 1.5 stance with hands together out in front of chest. Trainer pushes gently on client’s hands, making sure client doesn’t lose stability but uses enough tension to warrant strength adaptation. Client then maintains constant tension against trainer while decelerating into squat and standing back up. Client relaxes arms before beginning next rep.
Standing with feet shoulder-width apart, shift weight over right foot while shifting ViPR laterally to left. Squat to controllable range before holding bottom of squat. Now shift entire body over left foot while simultaneously shifting ViPR to right. Reverse sequence back to start position and then repeat on other side.
“Tradition is the illusion of permanence.”
As we scrutinize our current practices, it’s important not to “throw the baby out with the bath water.” During a squat, coaching cues such as “Keep the knees in line with the toes” or “Deep squats are bad for the knees” may still be proper for some in certain situations, but generalization of these coaching strategies has led to poor practice and widespread confusion about ideal human motion. Moving forward, we should continue to master the squat pattern with personalized recommendations based on each client’s unique anatomical structure, biomechanics and personal preferences.